1. Field of the Invention
The present invention relates to a ceramic diaphragm structure, and a method of producing the diaphragm structure, and more particularly to a ceramic diaphragm structure which has a thin, convex ceramic diaphragm portion that protrudes outwards, and exhibits excellent operating characteristics. In particular, the invention is concerned with a method of effectively controlling an amount of protrusion of the ceramic diaphragm portion, and such a ceramic diaphragm structure as obtained according to the method.
2. Discussion of Related Art
A diaphragm structure has been widely used for various kinds of sensors and the like, which structure has a substrate or base with at least one window or opening formed therethrough, and a thin diaphragm plate formed of a flexible material, for closing the window or windows thereby to provide a diaphragm portion or portions. In recent years, the diaphragm structure of this type has been increasingly used for piezoelectric/electrostrictive actuators. When the diaphragm structure is used as a component of a sensor, the sensor has appropriate means for detecting the amount of bending or flexural displacement of the diaphragm portion of the diaphragm structure, which displacement is caused by an object to be measured by the sensor. When the diaphragm structure is used as a component of a piezoelectric/electrostrictive actuator, the diaphragm portion of the structure is deformed or deflected by a piezoelectric/electrostrictive element formed on the diaphragm portion, so as to raise the pressure in a pressure chamber which is formed within the diaphragm structure.
The diaphragm structure as described above may be produced by forming an integral body which consists of the substrate serving as a base or support, and a film member (diaphragm plate) which is supported by the substrate and provides the diaphragm portion. For improved operating reliability, and increased heat and corrosion resistance, it has been proposed to form the diaphragm structure as a fired integral ceramic body, for use in a pressure sensor or a piezoelectric/electrostrictive actuator, as disclosed in U.S. Pat. No. 4,894,635 (corresponding to JP-A-63-292032) and U.S. Pat. No. 5,210,455 (corresponding to JP-A-5-49270) both owned by the assignee of the present application.
The integral ceramic diaphragm structure as described above is generally formed by laminating a ceramic green substrate having a window of a suitable shape, with a thin ceramic green sheet for closing the window, to thus form an integral green laminar structure having a diaphragm portion which closes the window at one of its opposite openings. The green laminar structure is then fired into the integral diaphragm structure. The inventors of the present invention found that the diaphragm portion, which is formed from the ceramic green sheet at its portion located above the window of the ceramic green substrate, may be concavely deformed or dented, or suffer from cracks, during the firing operation. The denting and cracking at the diaphragm portion are detrimental to the intended functions and operation of the diaphragm, resulting in reduced operating reliability.
The ceramic diaphragm structure as described above is normally provided with a flat diaphragm portion or portions. However, it is difficult to significantly increase the natural resonance frequency of such a flat diaphragm. Further, the flat diaphragm does not exhibit sufficiently high mechanical strength with respect to a force applied to the outer surface of the diaphragm, thus making it difficult to reduce the thickness of the diaphragm. Moreover, electrode films, piezoelectric/electrostrictive film and others formed on the flat diaphragm portion cannot be satisfactorily sintered.
To solve the above problems, the present inventors proposed a ceramic diaphragm structure as disclosed in a co-pending U.S. patent application Ser. No. 08/386,677, now U.S. Pat. No. 5,545,461, which includes a ceramic substrate having at least one window, and a ceramic diaphragm plate which is superposed on the ceramic substrate so as to close the window or windows. The ceramic substrate and the diaphragm plate are co-fired into an integral sintered body, such that the diaphragm plate includes at least one diaphragm portion which is aligned with the window(s). In the disclosed structure, each diaphragm portion protrudes outwards, in a direction away from the corresponding window, to form a convex shape. The convex diaphragm portion thus formed is free from denting and cracking, and exhibits an increased natural resonance frequency and high mechanical strength. Further, the convex diaphragm portion does not affect sintering of films (i.e., electrode films and piezoelectric/electrostrictive layer) formed on the outer surface thereof, assuring high operating reliability of the resultant ceramic diaphragm structure.
The convex shape of the diaphragm portion of the above ceramic diaphragm structure may be achieved by various methods, such as: a) controlling the sintering speed and shrinkage percentage of the diaphragm portion and the ceramic substrate, or b) applying a pressure to the diaphragm portion so as to protrude the diaphragm portion outwards, or c) making use of a difference in the coefficient of thermal expansion between the diaphragm portion and the substrate. In any of these methods, it is not easy to control the amount of protrusion of the diaphragm portion to a desired value without fail. Therefore, the amount of protrusion may fluctuate among a plurality of diaphragm portions formed in a single ceramic diaphragm structure, or among diaphragm portions of different diaphragm structures, resulting in a variation in the characteristics of the respective diaphragm portions.